You can configure everything from window style, to sound, to loading message, and much more

just a note, for the splash image and sound the url preference setting for local files must be like this:
(make sure the slashes are forward)
file:///D:/Downloads/Sounds/mail2.wav
or
file:///D:/Downloads/Sounds/splash.png

Mrtech helped my allot on this extension, thanks

WARNING: THIS IS AN UNSTABLE BETA, ONLY FOR USERS WHO WISH TO TEST THIS EXTENSION

It works fine for me, but the splash image takes about two seconds to load (not that much of a problem) then appears for two more seconds, disappears, and the browser takes about five more seconds to load. Is this intentional?

EDIT: Just looked at the code and saw that it is intentional. For a next release, I would think adding an event listener for the main window for loading would work much better than a simple timeout. I'm not sure if the window is actually loading that early in the startup process, but if it isn't, you could always set an interval to keep checking for it. (Unless there is a XPCOM component that can listen for a window opening event, which there probably is.)